This invention relates to a tunnel furnace partitioned into a plurality of zones with individual control of temperature and a method of making heat treatment of a plurality of, for example, glass or ceramic workpieces in the tunnel furnace.
Tunnel furnaces are widely used for heat treatment of various articles including glass and ceramic articles. The purpose of heat treatment may be firing, sintering or hot working such as bending of platy workpieces by utilizing softening of the heated workpieces. Usually workpieces to be passed through a tunnel furnace are placed on pallets, or molds in the case of heat bending, which are successively pushed into and out of the furnace. It is known, as shown in JP-A 47-25210 for instance, to partition a tunnel furnace into a plurality of zones by individually openable and closable shutters. Since each of the partitioned zones can be maintained accurately at a predetermined constant temperature it is possible to perform precision heat treatment of workpieces.
However, for heat treatment of glass or ceramic workpieces a tunnel furnace partitioned into constant temperature zones is not always convenient because in many cases it is necessary to raise or lower temperature in an intricate manner. For example, in the case of annealing workpieces of glass or a composite material containing glass it is necessary to perform gentle and precisely controlled lowering of temperature to suppress straining of the glass and relieve inevitably created some strains. If a zoned tunnel furnace of the aforementioned type is used for heat treatment of such workpieces the furnace must have many annealing zones each of which is maintained at a temperature only slightly lower than the temperature of the preceding zone. In the cases of ceramic workpieces attention has to be paid to the transformation points or transition points of the mineral materials of the ceramics because it is likely that rapid expansion or shrinkage occurs at each transformation or transition temperature and results in cracking or deformation of the workpieces. To prevent such phenomena it is necessary to interrupt heating or cooling of each workpiece at each transformation or transition temperature in order to maintain the workpiece approximately at that temperature for a length of time sufficient to uniform the temperature of every part of the workpiece. Therefore, the zoned tunnel furnace must include several zones specifically for maintaining the workpieces at the respective transformation or transition temperatures. Use of a tunnel furnace partitioned into too many zones is unfavorable for efficiency of the heat treatment operation.